This invention is related to the arts of growing photoconductive semiconductors from metallic solutions. Particularly, this invention provides a method of growing single crystals of silicon doped with a concentration of thallium, which thallium concentration is sufficiently high to give a high absorption coefficient. Thallium has a sufficiently large ionization energy to make extrinsic silicon detectors of 3-5 um infrared radiation which operate above 77K.
The growth of semiconductors from metallic solutions is well known. It is the basis for liquid phase epitaxy (LPE) processes, for example growth of gallium arsenide (GaAs) from gallium (Ga) solution. Thick layers can be grown by the solution growth (SG) process; for example, silicon (Si) can be grown from indium (In) solution. Both processes depend on the concentration of solute being different at different temperatures. In the LPE process a saturated solution is cooled, causing the solute in excess of the solubility limit at the lower temperature to precipitate out as an epitaxial layer. In the SG process, a temperature gradient is imposed across a molten solvent (e.g. In) such that more solute (e.g. Si) is soluble at one end than at the other. A source of solute is placed at the hotter end and a seed at the cooler end. The greater solubility at the hotter end will cause a concentration gradient to develop and the solute will diffuse down the concentration gradient to the cooler end where it will precipitate on the seed. The material grown will contain the solubility limit of solvent at the temperature at which the seed is maintained. In the example of Si grown from In solution, the Si grown will contain the solubility limit of In at the growth temperature. The present dopant used in Si for 3-5 um response is In. Indium has too small an ionization energy however, and therefore requires operation at &lt;60K. Thallium has a larger ionization energy in silicon than has indium, and has the advantage of a higher operating temperature.
The silicon-thallium system presents a unique problem. The solubility of silicon in thallium is infinitesimal even up to 1400.degree. C. so that silicon cannot be grown from thallium solution. However, silicon is soluble in tin (Sn) and can be grown from its solution. Tin is not electrically active in silicon; therefore, by adding tin to thallium we can grow silicon doped to the solubility limit of both tin and thallium. The inactive tin can be ignored while the thallium is a deep acceptor suitable for 3-5 um infrared detection. The novel feature is the addition of a second metal, tin, to the melt thallium, for single crystal solution growth. The unique feature of the tin is that it is not electrically active in silicon but dissolves enough silicon so that silicon can be grown from it.